Alpha-adrenergic mechanisms in myocardial ischemia.

alpha-Adrenoceptor-mediated effects of sympathetic activation on the heart and coronary circulation are reviewed with emphasis on the pathophysiology of myocardial ischemia. A classification of alpha-adrenoceptor subtypes is presented, and the effects of alpha-adrenoceptor activation on presynaptic sympathetic nerve terminals, cardiomyocytes, endothelium, platelets, and coronary smooth muscle cells are discussed. alpha-Adrenergic coronary vasoconstriction at rest and during situations of sympathetic activation such as exercise and excitement is analyzed for the segmental, transmural, and regional distribution of coronary blood flow. Evidence for a significant contribution of alpha-adrenergic coronary vasoconstriction to experimental and clinical myocardial ischemia is provided. Cardiomyocyte alpha-adrenoceptor activation may be involved in ischemic and reperfusion arrhythmias. The participation of presynaptic and postsynaptic alpha-adrenoceptors, as well as of alpha 1- and alpha 2-adrenoceptors, in experimental and clinical myocardial ischemia will require further investigation.